1. Field of the Invention
The present invention relates to a device for sup porting and linearly moving an object such as a scan mirror of an optical interferometer or a precision linear table, which must be positioned precisely.
2. Description of the Related Art
As is well known, a supporting device for supporting an object such as a scan mirror of an optical interferometer or a precision linear table is required to have a function for precisely positioning the object. To meet this requirement, a device for supporting and linearly moving an object comprises, in general, the mechanical elements: a movable member for supporting an object such as a scan mirror, a guide mechanism for linearly guiding the movable member smoothly, and a precise ball screw feed mechanism for moving the movable member.
In this linear movement/support device adopting the above mechanical positioning system, the structural parts must be machined with very high precision. Even if the parts are machined precisely, a problem of a ball screw, e.g. backlash, cannot perfectly be prevented. Thus, the linear movement/support device adopting the mechanical positioning system has a limit in positioning precision. In addition, when this linear movement/ support device is used in special environment (e.g. high temperature, high vacuum, etc.), lubrication of the ball screw may be deteriorated. Thus, this device cannot be used in the special environment for a long time.
In order to overcome the above problems, there has been proposed a linear movement/support device wherein an object which is moved by linear actuator is supported by a support spring. However, since the range of positioning for the object is limited by the resiliency of the plate spring, it is difficult to perform positioning in a wide range.
Further, in order to overcome the above problems, there has been proposed a linear movement/support device wherein a movable member is levitated by a magnetic force, i.e. a magnetic bearing, in a perfect non-contact manner. However, this device using the magnetic bearing has the following problem. In the linear movement/support device using the magnetic bearing, if the top portions of the stationary member-side magnetic poles face to the movable member-side, which magnetic poles are formed in projecting shapes, vibration of the movable member about its axis due to external disturbance can be passively damped by a magnetic attraction force acting between both magnetic poles. However, the vibration damping effect of such passive manner is low, and vibration may continue and a control system for the magnetic bearing may become unstable. Further, when the movable member must be positioned precisely, high precision cannot be maintained owing to the vibration.
Besides, in order to solve the above problems, it has been thought that new structural parts are added to construct a linear movement/support device wherein a fluid damper is used or mechanical friction is utilized According to this, however, the load capacitance of the linear movement/support device itself lowers.
This type of linear movement/support device has more problems. Specifically, the center of gravity of the movable member moves relative to the magnetic bearing in accordance with axial movement of the movable member, with the result that a magnetic support control system becomes unstable. Further, when the stationary member for supporting the movable member is vibrated owing to external vibration, the movable member makes a compound movement of movement of its center of gravity and rotational movement about the center of gravity. Thus, the positioning precision cannot be maintained.
Furthermore, this type of linear movement/support device has the following problem. Regarding the magnetic bearing, it is necessary to detect a radial position of the movable member by means of a position detector and feed the detection output back to a levitation control system, thereby controlling the levitation position. Thus, the precision of the position of the movable member in the case of linearly moving the movable member over a long distance is determined by the machining precision of position-detecting parts such as position detecting on surfaces. Since the machining precision of the parts is limited, the precision of, e.g. directivity of a scan mirror of an optical interferometer is limited by the machining precision When the movable member is supported by the magnetic bearing, all six degrees of freedom of the movable member must be controlled, resulting in complexity of control circuits.